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DMX & No Dimmer

Discussion in 'Lighting and Electrics' started by beam_1973, Mar 9, 2006.

  1. beam_1973

    beam_1973 Member

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    Apologies if this is covered in one of the numerous DMX threads I started going through, but my question is two fold:-

    1. Can someone advise if its is possible to run DMX signal direct from a lighting desk to DMX controlled fixtures (rather than from the desk and via the DMX in/out sockets on a dimmer rack); and

    2. If this is possible, is there a limit to the length of DMX cable run between desk and the fixture, without affecting DMX signal strength (and therefore fixture operation).

    Any help greatly appreciated!!

    Craig (first time question asker)
     
  2. soundlight

    soundlight Well-Known Member

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    Answer to the first question: YES! DMX is just a control protocol, it doesn't have to go through a dimmer rack to operate moving lights. Just hook it straight up.

    Non-answer to second question: I don't know the cable length limit, but if you are going to do long cable runs or daisy chain large numbers of fixtures, you want to make sure to get a DMX terminator on the "out" DMX port of the last fixture.
     
  3. CURLS

    CURLS Member

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    To answer question one: You can either run straight into a "intelligent" fixture or dmx device straight out of your console, or you can visit the distro as well (as long as its wired correctly)

    Q2: The cable can be up to 100' or even 250' in length or possibly even more from console to your first electric. However, once you get there you may want an iso splitter to boost your signal back up again. As previously mentioned you do want to terminate to prevent mirroring signal, 120Ohm resistor between pins 2 and 3 of either a 3 pin or 5 pin xlr.

    If this doesnt answer let me know!
     
  4. Footer

    Footer Senior Team Senior Team Premium Member

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    DMX can run 1500' without be regenerated.... and its a 100ohm resistor....
     
    Last edited: Mar 10, 2006
  5. fosstech

    fosstech Active Member

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    You got me scared there for a second ;). I can guarantee you that there is more than 250' of DMX wire running from the console to the dimmer rack in my space, and it works great...
     
  6. BillESC

    BillESC Well-Known Member Premium Member

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    1000' of DMX cable is not unusual in a large show situation. Make sure to use high quality DMX specific cable such as Duraflex for such applications.
     
  7. ship

    ship Senior Team Emeritus Premium Member

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    urr, it's a 120ohm resistor I remember though a 100ohm resister would probably work just fine. Perhaps it's a 100ohm and 120ohm I remember for special equipment.

    Can someone verify this all?
     
  8. ship

    ship Senior Team Emeritus Premium Member

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    DMX protocol cable would be the key in insuring your data short of opto or iso splitter gets the signal to the gear. You can send your data thru the rack than to the gear or send it where you please. Once you assign a channel your dimmers while on a different channel won't mis-read the channel, nor will the gear in the air unless assigned to the same channel.

    250' is not a lot to worry about unless using crap data cable and even than probably not much about. It is a fairly robust signal to some extent at least.
     
  9. beam_1973

    beam_1973 Member

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    thanks all for your replies (I just wasn't sure whether the dimmer provided a "signal boost" or not) ... most helpful and will come in very handy!!

    Cheers
     
  10. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    From what I have read 120ohm is the value most frequently mentioned and certainly what all of my terminators use. Although, I am not sure that a difference of 20ohms is going to be a major factor and you could quite possibly use one slightly higher.

    A former member here that made his living from designing and selling protocol converters also quoted the value to be 120 ohms.

    I just did a search for the topic of DMX termination and found it HERE.

    However, having said that, the fact that DMX is a robust signal and will quite happily function on less than optimal cable with little or no obvious signs of degradation that as long as the value of the resistor is around 120 ohms then you will be pretty close to optimal.
     
  11. Footer

    Footer Senior Team Senior Team Premium Member

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    I got that number from John Huningtons books....
     
  12. beam_1973

    beam_1973 Member

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    Regarding termination ... if the final fixture in the DMX loop has a dipswitch available that you can flick on to "terminate" the signal, is this considered acceptable practice, or should I still be using a physical terminator as well (in the last "DMX Out" I presume) .... I have started using 6 DMX colour changers (terminate dipswitch turned on in the last unit) along with a bunch of non-intelligent fixtures and I don't seem to be experiencing any mirroring problems (not that I know what they look like anyway!!). Perhaps its a "if it ain't broke, don't fix it situation"???
     
  13. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Yes - this does the same thing. Older units and even some new ones don't have this feature, so you have to plug one into the DMX out of the last unitin the chain.
     
  14. CURLS

    CURLS Member

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    The termination on the last fixutre is good enough no sense doin it twice or wasting a terminator on a fixture that can already do it. (a resistor is a resistor) If you don't notice any problems as of yet, stick with the if it ain't broke dont fix it rule! But, mirroring problems can cause fixtures to do the same thing or fixtures to kinda freak out. Hard to explain but you would know it when you have the problem. I would more or less say don't worry about it if all your cabling is good and everything.
     
  15. beam_1973

    beam_1973 Member

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    Thanks Mayhem and Curls ... much appreciated! :)
     

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